Engines may be configured with an exhaust heat recovery system for recovering heat from an internal combustion engines exhaust energy. The heat is transferred from the hot exhaust gas to a coolant through an exhaust gas heat exchanger system. The heat from the coolant, circulated through the heater core, may be utilized for functions such as heating the cylinder head thus warming the passenger cabin quickly, thereby improving engine efficiency. In hybrid electric vehicles, the recovery of exhaust heat improves fuel economy by warming engine temperatures quicker, thereby allowing for a faster engine shut-off and extended use of the vehicle in an electric mode.
Exhaust heat may also be retrieved at an exhaust gas recirculation (EGR) cooler. An EGR cooler may be coupled to an EGR delivery system to bring down the temperature of recirculated exhaust gas before it is delivered to the intake manifold. EGR is used to reduce exhaust NOx emissions, improve fuel economy by reducing the throttling losses at low loads, and improve knock tolerance.
During cold start conditions and/or during engine idling, catalyst temperature may be low, for example lower than a light-off temperature. Due to low temperature, catalysts may not be able to effectively convert cold-start exhaust hydrocarbons. Hydrocarbon (HC) traps, such as those containing Zeolite, may be used to adsorb HC from a low temperature exhaust. The HC traps need to be purged periodically to remove the accumulated HC.
Several approaches are provided for trapping hydrocarbons exiting the catalyst during cold start conditions. In one example, as shown by Lupescu et al. in U.S. Pat. No. 8,635,852, a HC trap may be coupled to a bypass passage parallel to the main exhaust passage. During cold start conditions, exhaust may be routed through the HC trap before being released to the atmosphere through a tailpipe. Cold-start hydrocarbons may be adsorbed at the HC trap. After engine warm-up, hot exhaust is routed in an opposite direction through the HC trap, purging the trapped hydrocarbons to the engine intake manifold for combustion.
However, the inventors herein have recognized the above issues and potential disadvantages with the above approach. As one example, due to the position of the HC trap, exhaust entering the trap may not be cold enough for substantially all cold-start hydrocarbons to be adsorbed, leading to degraded exhaust emissions. In addition, water from the cold exhaust may condense on the HC trap, decreasing the HC storing functionality of the zeolite. The inventors have also recognized that it may be difficult to coordinate EGR cooling with exhaust heat recovery in such a system. In particular, the heat recovered at the EGR cooler cannot be effectively used for heating a cabin space due to low heat flux at lower mass flows. As a result, a distinct heat exchanger is required for cabin heating. Likewise, even if heat is extracted from exhaust at a heat exchanger, the cooled exhaust is not recirculated, resulting in the need for a distinct EGR cooler. The additional components add cost and complexity as well as difficulty in achieving the desired heat transfer for the distinct operations.
The inventors herein have identified an approach by which the issues described above may be at least partly addressed. One example method for an engine comprises: operating in a first mode with exhaust gas from do stream of an exhaust catalyst flowing through an exhaust bypass, and in a first direction through each of an upstream heat exchanger and a downstream hydrocarbon trap coupled in the exhaust bypass, and then to an exhaust tailpipe; and operating in a second mode with exhaust flowing from downstream of the exhaust catalyst through an exhaust passage, then in a second, opposite direction through the hydrocarbon trap and then through the heat exchanger, and then to the engine intake. In this way, cold start HC emissions can be reduced while recovering exhaust heat for cabin heating.
In one example, an engine system may be configured with a heat exchanger positioned downstream of a catalytic convertor in an exhaust bypass disposed parallel to a main exhaust passage. A hydrocarbon trap (e.g., zeolite based trap) may be positioned in the bypass downstream of the heat exchanger. A diverter valve may be used to enable exhaust to be diverted from downstream of the catalytic converter into the bypass, and through the heat exchanger and HC trap in one of two directions, a position of the diverter valve adjusted based on engine operating parameters. For example, during cold start conditions, the valve may be adjusted to flow exhaust through the bypass passage in a first direction through the heat exchanger, then through the HC trap, and then on to the exhaust tailpipe. During the flow in the first direction, exhaust heat is transferred to the heat exchanger and the cooled exhaust may flow through the HC trap wherein the hydrocarbons may be adsorbed. At the heat exchanger, the heat from the exhaust may be transferred to a coolant circulating and the hot coolant may then be used for functions such as cabin heating. By flowing the exhaust through the heat exchanger and then through the HC trap, condensate generated in the exhaust is delayed from entering the HC trap, enabling the exhaust HCs to be stored in the HC trap with a higher efficiency. In comparison, after engine warm up, the HC trap may be purged by adjusting the valve to flow hot exhaust through the main passage and then into the bypass, the exhaust flowing through the bypass in a second direction opposite to the first direction (through the HC trap and then through the heat exchanger) before the exhaust is recirculated to the engine intake via an EGR passage. The hot exhaust may purge the HC trap and the residuals may be routed to the engine intake manifold with EGR. The heat recovered at the heat exchanger during this flow is transferred to the circulating coolant, and thereon to a heater core for further use, such as in heating a passenger cabin and/or heating a cylinder head. Further, during conditions when coolant warm up is required and EGR is not required, the valve may be adjusted so that a portion of the exhaust can pass through the HC trap and heat exchanger before being returned to the main exhaust passage for release to the atmosphere. At the heat exchanger, heat from the exhaust may be transferred to the coolant thereby increasing coolant temperature.
In this way, the heating requirements of an engine system may be met using a single heat exchanger while reducing the emission of cold-start hydrocarbons. By flowing cold-start exhaust through a heat exchanger and then through a HC trap before releasing the exhaust through a tailpipe, the heat exchanger may delay the condensation of water in the cold exhaust from traveling to the downstream HC trap. As such, this delay allows the cold-start HCs to slip past the heat exchanger to the downstream HC trap, allowing the trap to be used substantially exclusively for exhaust HCs while the competing water is retained at the heat exchanger. At the same time, heat released at the heat exchanger can be advantageously used for heating a vehicle cabin space. Then, once the main catalyst has been warmed to light-off levels, the trapped HCs can be purged to the engine intake as EGR that is cooled at the heat exchanger. By providing the functions of an EGR cooler and an exhaust heat exchanger via a single heat exchanger, cost and component reduction benefits are achieved without limiting the functionality or capability of either system. In addition, by adding a HC trap downstream of the heat exchanger, cold start hydrocarbons may be effectively adsorbed. The technical effect of positioning the HC trap downstream of the heat exchanger is that water from the exhaust would condensate on the heat exchanger and water condensation on the Zeolite may be avoided thereby improving its efficiency. By using a diverter valve to regulate the flow of exhaust through a bypass passage, exhaust can be flowed in both directions across the heat exchanger and the HC trap. As such, this improves the heat transfer efficiency and facilitates HC trap purging. Overall, by improving the amount of waste heat that can be recovered from exhaust using fewer components, engine fuel economy and performance is improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.